Dealing with the EDGE Evolution

With spectrum issues plaguing the US sector and European carriers looking for ways to supplement wideband CDMA (W-CDMA) services, the EDGE air interface has once again taken center stage as the next evolution path for wireless networks.

Unlike GPRS, which required a total rework of the network, EDGE promises to deliver higher capacity and faster data services to wireless networks through a change only on the radio interface. But while the EDGE spec seems like a simple transition on paper, in reality EDGE comes equipped with new modulation schemes, multi-slot operation, and a host of other advanced radio design elements. Here's a look at some of the new capabilities that EDGE delivers and the impact they will have on wireless system and network designs.

The OverviewThe objective of EDGE is two-fold: 1) increase data throughput while limiting impact on an existing GSM/GPRS network, and 2) enable the ability for operators to offer revenue-generating differentiated levels of services to end user through the negotiation of quality-of-service (QoS) parameters in all the interfaces of the EDGE infrastructure network.

As illustrated in Figure 1, EDGE shares the radio equipment and the data core of a GSM/GPRS network. An advanced EDGE implementation allows for the multiplexing of EDGE and GPRS services on the same radio timeslots, enabling EDGE capabilities without the need for additional RF spectrum.

Figure 1: Architecture for a typical GSM/GPRS/EDGE system

EDGE enhances the data throughput by a combination of different techniques:

EDGE's 8-level phase-shift keying (8PSK) modulation scheme allows a raw radio throughput increase of 3 times the raw radio throughput of GPRS with the same number of timeslots.

Nine new coding schemes using both Gaussian minimum shift keying (GMSK) and 8PSK modulations and a link adaptation algorithm are introduced to automatically switch between coding schemes based on the radio environment.

The incorporation of incremental redundancy or automatic repeat request (ARQ) type II improves the effectiveness of data retransmissions in the case when the data are received in errors.

The GPRS radio link controller/media access controller (RLC/MAC) layer has been improved for EDGE, allowing for an increase of the end user performance.

Let's look at each of these in more detail starting with the 8PSK modulation scheme.

8PSK is Enough
The EDGE standardization leads to changes in the physical layer. In addition to existing GMSK modulation used in GSM/GPRS, EDGE uses 3π/8-shifted 8PSK modulation (Figure 2).

Constellation diagram of 8PSK modulation.

Through the 3π/8-shifted approach, the EDGE modulator will rotate the phase of each modulating symbol by multiples of 3π/8 during every symbol period. 8PSK modulation has eight distinguishable symbols and each symbol represents 3 bits of information. Alternatively, GMSK modulation contains one bit of information per symbol, so EDGE increases by three the transmission bit rate over GPRS.

Table 1 compares the maximum symbol rates and data rates between 8PSK and GMSK modulation as implemented in EDGE and GPRS.

Table 1: Technical Data for GMSK and 8PSK

Modulation

GMSK

8PSK

Symbol Rate

270 ksymbols/s

270 ksymbols/s

Bits per Symbol

1 bit per symbol

3 bits per symbol

Modulation Bit Rate

270 kbits/s

810 kbits/s

Max Users Data Rate per TS

20 kbit/s for CS4

59.2 kbit/s for MCS9

Unlike GMSK modulation, which has a constant envelope signal, 8PSK modulations use linear modulation techniques with amplitude varying signal. An EDGE 8PSK-modulated signal has a peak-to-average ratio (PAR) of 3.2 dB and a peak-to-minimum signal ratio of 17 dB.

One of the challenges in EDGE design is to implement a cost effective, highly linear modulator and power amplifier that can handle the peak to average power ratio. Insufficient linearity in the modulator and power amplifier designs will result in a loss of EDGE throughput due to the necessity to operate at reduced amplifier output power.

In a practical deployment, where EDGE is situated on the same TDMA signal as the base control channel (BCCH), a reduced 8PSK power would result in undesirable mobile performances and adversely impact features such as cell selection / re-selection. It is therefore important to closely match the average power for both the GMSK and 8PSK modulation.

Link Adaptation
The EDGE air interface introduces nine modulation coding schemes (MCS), each employing different data coding (user payload) and channel coding (header and protection) rates with GMSK and 8PSK modulations (see Figure 3). Each MCS is designed to deliver the optimal throughput under different radio environments [carrier-to-interference ratio (C/I) and carrier-to-noise ratio (C/N)]. It will be possible through the link adaptation feature to change the modulation and coding scheme during the communication to adapt to the changing radio environment. This enables a more efficient use of the spectrum and improves performance and robustness.

Figure 3: EDGE is equipped with nine modulation coding schemes.

As illustrated in Figure 3, EDGE coding scheme MCS-4 sports a user payload size that is a multiple of MCS-1. Likewise, MCS-2, MCS-5 and MCS-7 have payload sizes that are multiples of MCS2; and MCS-3, MCS-6 and MCS-9 have payload sizes that are multiples of MCS-3. These groupings of EDGE MCS are called "families."

MCS-3, MCS-6 and MCS-8 are also considered one family when the user payload of MCS-3 and MCS-6 are reduced enabling multiple blocks of these coding schemes to fit in one block of MCS-8. The reduced payload of MCS-3 and MCS-6 will need to be padded before coding when transmitting as a member of this family.

Table 2 illustrates the organization of MCS into families. With these configurations, EDGE allows for the re-segmentation of the original data block for retransmission using a different coding scheme than the original data block's MCS. However, the new MCS chosen for the retransmission is limited to one of the member of the same family of the originally transmitted data block's MCS.

Table 2: MCS Families

Family Name

Modulation Coding Schemes

User Payload (octets)

A

MSC-3, MSC-6, MSC-9

37, 2x37, 4x37

A with padding

MCS-3, MCS-6, MCS-8

34+padding, 2x (34+padding), 4*34

B

MCS-2, MCS-5, MCS-7

28, 2x28, 4x28

C

MCS-1 and MCS-4

22, 2x22

The network automatically adapts the coding scheme according to the radio link quality. This adaptation is done automatically for the downlink, and the network commands the mobile station (mobile phone) to use the optimum MCS for the uplink. In order to perform this adaptation, the mobile station (for downlink transfer) and the base transceiver station [BTS] (for uplink transfer) carry out the radio link quality measurements (LQM), which are transmitted to the packet control unit service node (PCUSN). According to the radio link quality, the PCUSN can trigger a modification of the coding scheme.

In EDGE, the LQM consist of the mean bit error probability (MEAN-BEP) and the coefficient of variance of the bit error probability (CV-BEP). The measurements are performed on every radio block and can be reported back to the network for filtering and triggering of MCS changes with improved reactivity and accuracy compared to GPRS.

The link quality measurements provide an excellent indication of the radio environment and, in particular, the C/I and C/N. With the knowledge of the estimated C/I, the networks can trigger MCS adaptation to deliver the maximum throughput for the environment. Figure 4 illustrates the mapping of C/I to MCS and achievable throughput.

Figure 4: Throughput versus C/I for each MCS.

As indicated in Figure 4, MCS-9 does not provide the highest throughput in the entire range of C/I. In fact, each MCS is designed to deliver the optimal throughput in a range of C/I. In this respect, link adaptation (LA) provides the mechanism to optimize the end user throughput by selecting the most appropriate MCS scheme for the given radio conditions.

To ensure a certain level of throughput at the cell edge, the network must provide the required level of C/I at cell edge. There would also be a similar requirement for C/N. Therefore, if the current network C/I and C/N design does not meet the required level of performance at the cell edge, then changes in site density and frequency planning are required to achieve the desired throughput. In practice, the network design and frequency plan would remain constant, and EDGE throughput at the cell edge and aggregate throughput throughout the entire cell will be based on the C/I and C/N distribution achieved with the current network design.

Incremental Redundancy
EDGE also makes use of an enhancement of the classical ARQ scheme, known as Hybrid ARQ of type II or incremental redundancy (IR).

In a classical ARQ scheme, like the one used in GPRS, the data at the transmitter is split into consecutive blocks. In each block, the user data are encoded by adding some redundancy using convolutional coding, and completed by a header and trailer. A cyclic redundancy check (CRC) checksum is added in order to guarantee the integrity of the received data after decoding.

The receiver decodes the received block, then computes the checksum if the computed checksum is equal to the decode checksum, and the data block has been correctly received. If it has not, the block is discarded and the receiver provides feedback to the transmitter using acknowledgment (acknowledgment bitmaps in GPRS) that will signal the transmitter to retransmit the incorrectly received data blocks as previously coded.

In IR, the redundancy added to the data is not the same at each retransmission. Depending on the coding scheme, the block coding process cycles through two or three puncturing scheme choices during successive repetitions of the same block. Puncturing is a technique of removing bits in predetermined locations of the data block after the block has been channel coded. This allows the data blocks to fit in the allocated user payload space. Channel coding before puncturing adds redundancy to the bits of the data block; therefore, the receiver in the decoding process can recover the punctured bits.

On the receiving side of an IR implementation, if the decoding failed, the receiver keeps the soft bits of the inadequately received block in memory. It can then reuse these soft bits by combining them with the next retransmitted block's soft bits and increase the chances for successful decoding. Due to the special design of the redundancy used, the probability to correctly receive a block in a small number of retransmissions is then increased significantly compared to GPRS.

Only blocks with the exact same coding format can be combined for IR. Therefore, retransmission using block re-segmentation to adapt to a lower MCS cannot work with IR. In this case, the soft bits of the unsuccessfully decoded block stored in memory are discarded and the retransmitted block is decoded without IR combining. Block re-segmentation for retransmission can only occur using a MCS in the same family.

As Figure 5 points out, IR increases the throughput performance of each MCS for a given C/I. However, even with IR, MCS9 still cannot deliver the highest throughput performance in all the C/I ranges. Combining IR for retransmissions and LA to choose the optimal MCS for new transmissions will produce the best throughput performance with minimal additional back haul requirements.

RLC/MAC Improvement
The RLC/MAC layer is significantly improved with EDGE. In GPRS, the acknowledgement window used in the ARQ mechanism is limited to 64 RLC/MAC blocks. This limited window can quickly result in a stalling condition with handsets that support multiple time slots. The stalling condition occurs when an incorrectly received block has not been acknowledged before 64 new blocks have been transmitted. During the stalling condition, the transmitter stops transmitting new blocks until this oldest, unacknowledged block is successfully transmitted.

Handsets capable of multiple time slots are more susceptible to this stalling condition because the 64-block buffer can quickly reach its limit with the increase in data rate capability. In EDGE this acknowledgement window size has been extended up to 1024 blocks depending on the MCS selected. This increase has significantly reduced the stalling window condition even for multiple time slots handset operating at higher MCS.

Enhanced QoS
EDGE is part of the Third Generation Partnership Project (3GPP) Release 99 (Rel99) specifications, which provides quality of QoS) enhancements to a wireless system. These include:

Rel99 introduces new QoS parameters in the QoS profile, which allows a differentiation between different types of traffic (conversational, streaming, interactive and background).

Rel99 also introduces the concept of packet flow context (PFC) and associated procedures (creation, modification, and deletion), which result in the negotiated QoS for a PDP Context. In particular, this allows the base station system (BSS) to be involved in the QoS negotiation. At creation of the PDP (BSS packet flow creation procedure) or during a session (BSS packet flow modification procedure), the PCUSN can request a specific QoS based on the resources availability in the BSS.

Based on these enhancements, Rel99 offers a real end-to-end QoS solution for GPRS/EDGE that is comparable to the way UMTS handles resources.

Wrap Up
While the tug of war between access technologies - CDMA vs. UMTS vs. GSM -- continues to be debated globally, EDGE provides an ideal solution for GSM carriers to add data capacity using limited spectrum. When you consider the fact that GSM supports more subscribers today than any other access technology (roughly 65 to 70% of the global subscriber market), and that GSM/GPRS operators are scrambling to add capacity to support user growth and launch next generation data services, the increased capacity and throughput offered by EDGE becomes very compelling. And, in a market where wireless carriers must squeeze the most out of capital outlayspast and futureit is no real surprise that we are seeing a renewed wave of interest in EDGE from our GSM customers.

About the AuthorsTerry Locke is an EDGE team leader in Nortel's Wireless Networks Division. He holds a BScEE and MEng from the University of Saskatchewan, and an MBA in from University of Calgary. Terry can be reached at tlocke@nortelnetworks.com.

Si Nguyen is the BSS Systems Advisor in Nortel's Wireless Networks division. He holds a Bachelor of Science in Electrical Engineering from Carnegie Mellon University and can be reached at siquocn@nortelnetworks.com.

Dominique Moreuil is the manager of Nortel Networks' GPRS/EDGE Access Product Line Management group. He holds a D.E.S.S. Diploma in Electronic Systems from the University of Orsay Paris XI. Dominique can be reached at dmoreuil@nortelnetworks.com.